A series of experiments are in progress employing chromosome selective staining techniques to study the molecular organization of human chromosomes. Prefixation treatments with the synthetic polyanion polystyrene sulfonate removes specific histone fractions sequentially from calf thymus chromatin and alters the quinacrine banding pattern, but not the Giemsa banding pattern; of human chromosomes. The role of specific histone fraction in the production of bands by modified Giemsa banding techniques and fluorochrome stains is being investigated. Giemsa banded chromosomes have been obtained following in vivo, in vitro or in situ exposures of mouse or human cells to the DNA alkylating agents ethyl methanesulfonate, methyl methanesulfonate and methyl ethanesulfonate. The banding patterns induced by DNA alkylating agents are being compared to the Giemsa banding patterns of human chromosomes stained by the urea and acetic-saline-Giemsa banding techniques. The relationship between chromosome bands and chromosome coils is being investigated by comparing the Giemsa and fluorescent bands of mitotic chromosomes to the chromomere patterns of meiotic chromosomes and the coiling pattern induced in mitotic chromosomes by special pretreatments.